In order to successfully exploit subterranean hydrocarbon reserves, information about the subsurface formations and formation fluids intercepted by a wellbore is generally required. This information may be obtained via sampling formation fluids during various drilling and completion operations. The fluid may be collected and analyzed, for example, to ascertain the composition and producibility of hydrocarbon fluid reservoirs.
Downhole sampling tools commonly include a fluid entry port (or probe), a fluid inlet valve, and one or more sample chambers. Formation fluids may be pumped (e.g., using a reciprocating positive displacement pump) through fluid analysis instrumentation into a sample chamber. Such pumping methods work well with incompressible (or nearly incompressible) fluids such as those containing primarily liquid water and/or oil. However, when the formation fluid is highly compressible (such as with a gaseous fluid), positive displacement pumps tend to be inefficient as much of the stroke volume compresses and decompresses the gaseous fluid rather than pumping the fluid. Repeated compression and decompression may also result in irreversible changes to the formation fluid which compromises the integrity of the sample.
Gaseous formation fluids may also be dumped (or received) into a sample chamber using the formation pressure to drive the fluid into the chamber. However, such methodologies tend to significantly reduce the pressure of the sample, resulting in a low pressure, low mass sample. These methods can also cause irreversible changes to the fluid owing to expansion of the gas into the sample chamber.
Therefore there is a need in the art for improved formation fluid sampling tools and methods, particularly for obtaining samples of highly compressible fluids such as gases.